(1) Field of the Invention
The present invention relates to a thermal head for thermal recording. More particularly, the present invention relates to a thermal head having a protecting film which is excellent in the abrasion resistance, the oxygen barrier property and the barrier property against injected ions.
(2) Description of the Prior Art
A thermal head is used as a recording device of an apparatus such as a facsimile apparatus or a printer and attains an effect of reducing the size and weight of the apparatus, a maintenance-free effect, an effect of improving the reliability of the apparatus and an effect of reducing the price of the apparatus, and the thermal head makes a great contribution to development of the apparatus.
A conventional thermal head has a structure in which a heat-generating resistor composed of tantalum nitride (Ta.sub.2 N) or the like and an electrode composed of aluminum (Al) or copper (Cu) are laminated in sequence on a substrate composed of an electrically insulating material such as alumina or glass, and a certain electric power is applied to the heat-generating resistor through the electrode to generate Joule heat and exert the function of the thermal head.
In this conventional thermal head, a protecting film having a two-layer structure composed of silicon oxide (SiO.sub.2) and tantalum pentoxide (Ta.sub.2 O.sub.5) or a protecting film having a single layer structure composed of silicon oxynitride (SiN.sub.x O.sub.y ; 1.33&gt;x&gt;0 and 2&gt;y&gt;0) is formed on the outer surfaces of the heat-generating resistor and electrode to protect the heat-generating resistor and electrode from the abrasion by contact with a heat-sensitive recording paper or heat transfer ribbon and also from oxidation by contact with oxygen in air.
In this conventional thermal head, however, in case of a protecting film having a two-layer structure composed of silicon oxide (SiO.sub.2) and tantalum pentoxide (Ta.sub.2 O.sub.5), tantalum pentoxide (Ta.sub.2 O.sub.5) is relatively low in the hardness and inferior in the abrasion resistance and silicon oxide (SiO.sub.2) is insufficient in the oxidation-preventing property and is chemically unstable. For example, silicon oxide reacts with a heat-sensitive substance on the heat-sensitive recording paper to drastically degrade the abrasion resistance, with the result that the abrasion by sliding contact with the heat-sensitive recording paper is great in this protecting film and the thermal head cannot be used for a long time. Furthermore, the heat-generating resistor and electrode are oxidized and corroded by oxygen in air, and the resistance value of the heat-generating resistor is changed or the electrode line is broken, with the result that uneven printing density or insufficient printing is caused and the printing performance is drastically degraded.
In case of a protecting film having a single layer structure of silicon oxynitride (SiN.sub.x O.sub.y ; 1.33&gt;x&gt;0 and 2&gt;y&gt;0), since silicon oxynitride has a high hardness and is excellent in the oxidation resistance, the abrasion by sliding contact with a heat-sensitive recording paper is reduced and the oxidative corrosion of the heat-generating resistor and electrode is effectively prevented. However, since ions are readily diffused in this silicon oxynitride, when the protecting film is brought into sliding contact with a heat-sensitive recording paper or a heat transfer ribbon, Na.sup.+ ions or K.sup.+ ions contained in the heat-sensitive recording paper or heat transfer ribbon are diffused in the protecting film, and these ions act on electrodes, especially earth electrodes, to cause breaking of electrode lines, with the result that supply of electric power to the heat-generating resistor becomes impossible and the function of the thermal head is compeltely lost.